US6950562B2 - Data processing method - Google Patents

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US6950562B2
US6950562B2 US10/028,100 US2810001A US6950562B2 US 6950562 B2 US6950562 B2 US 6950562B2 US 2810001 A US2810001 A US 2810001A US 6950562 B2 US6950562 B2 US 6950562B2
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processing method
data processing
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US20020126912A1 (en
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Laurent Jean Marie Rouvellou
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Koninklijke Philips NV
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
    • H04N19/85—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using pre-processing or post-processing specially adapted for video compression
    • H04N19/86—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using pre-processing or post-processing specially adapted for video compression involving reduction of coding artifacts, e.g. of blockiness
    • H04N19/865—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using pre-processing or post-processing specially adapted for video compression involving reduction of coding artifacts, e.g. of blockiness with detection of the former encoding block subdivision in decompressed video
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
    • H04N19/50—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using predictive coding
    • H04N19/503—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using predictive coding involving temporal prediction
    • H04N19/51—Motion estimation or motion compensation
    • H04N19/527—Global motion vector estimation
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
    • H04N19/80—Details of filtering operations specially adapted for video compression, e.g. for pixel interpolation
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
    • H04N19/85—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using pre-processing or post-processing specially adapted for video compression
    • H04N19/86—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using pre-processing or post-processing specially adapted for video compression involving reduction of coding artifacts, e.g. of blockiness

Definitions

  • the present invention relates to a method of processing data contained in a digital input image.
  • the invention finds its application in the detection of blocks in a previously coded and then decoded digital image according to a block-based coding technique, for example, the MPEG standard (Motion Pictures Expert Group) and in the correction of the data comprised in these blocks for attenuating the visual artifacts caused by the block-based coding technique.
  • a block-based coding technique for example, the MPEG standard (Motion Pictures Expert Group)
  • MPEG standard Motion Pictures Expert Group
  • European patent application no. 0 817 497 A2 describes a method which permits to reduce blocking artifacts and artifacts due to the ringing noise of a motion-compensated image.
  • the processing method according to the prior art comprises a gradient filter step which permits to generate a binary edge map on which global thresholding and local thresholding are performed.
  • Said method comprises a step which permits to decide whether an area inside the binary edge map, which area is determined with the aid of a filter window, is a homogeneous area or a edge-containing area.
  • the method finally comprises a filter step which utilizes a first set of predetermined coefficients if the area is homogeneous, and a second set of predetermined coefficients if the area contains edges, the second set of predetermined coefficients being adapted as a function of the position of the contours in the area.
  • the invention takes the following considerations into account.
  • the data processing method according to the prior art requires the calculation of an average and of a standard deviation of gradient values of the pixels belonging to a block for each block of the digital input image. Such a calculation is costly in terms of calculation resources, which renders such a method complex to implement.
  • the data processing method according to the present invention is characterized in that it comprises a step of detecting blocking artifacts which originate from a block-based coding technique, said detection step comprising the sub-steps of:
  • Such a data processing method permits to detect the blocking artifacts on the basis of a simple continuity test, which blocking artifacts originate from the block-based coding technique.
  • This continuity test as well as the step of determining an artifact value require only a limited number of pixels, that is, the current pixel and several pixels around the current pixel.
  • the discontinuity value is computed from the comparison between a value of a current pixel and a half-sum of the values of the pixels immediately preceding and following the current pixel.
  • the data processing method according to the invention consequently requires only minor calculation resources for detecting the blocking artifacts.
  • the processing method according to the prior art does not differentiate the block borders which correspond to blocking artifacts from the ones that correspond to the natural object contours contained in the image.
  • a post-processing of the block borders detected by such a method thus risks to degrade the natural contours contained in the image.
  • the data processing method according to the invention further comprises a gradient filter step of filtering the values of the pixels, suitable for detecting a natural contour area in the digital input image.
  • the pixels coming from the blocking artifact detection step will be processed with the exception of the pixels contained in the natural contour areas determined by the gradient filter step.
  • the data processing method is characterized in that it comprises the sub-steps of:
  • Determining the size and position of the grid corresponding to the blocks of the block coding technique permits to apply the post-processing step in adapted form without the need to know the decoding parameters.
  • FIG. 1 is a diagram representing the data processing method according to the invention
  • FIGS. 2 a and 2 b illustrate the step of determining block artifacts for various configurations of pixel values
  • FIG. 3 is a diagram representing the steps of identifying and storing block artifacts originating from the block-based coding technique in a horizontal direction
  • FIG. 4 is a diagram representing the identification and block artifact storage steps originating from the block-based coding technique in a vertical direction
  • FIG. 5 is a diagram representing the step of calculating the position of the grid
  • FIG. 6 is a diagram representing the step of calculating the size of the grid
  • FIGS. 7 a and 7 b illustrate the gradient filter step, which permits to detect a natural contour area in a horizontal and vertical direction, respectively, and
  • FIGS. 8 a and 8 b illustrate the low-pass filter step, which permits to process the block artifacts originating from the block-based coding technique in a horizontal and vertical direction, respectively.
  • the present invention relates to a method of processing data which are contained in a digital video input signal, said method being intended for improving the visual quality of said digital video signal when the latter has previously been coded according to a block-based coding technique.
  • the data processing method has been developed more particularly in the framework of coded and then decoded digital image sequences according to the MPEG standard. The method nevertheless remains applicable for any other digital video signal which has been coded and then decoded according to a block-based coding technique such as H.261 or H.263, for example.
  • FIG. 1 represents a diagram of the data processing method according to the invention. Said data processing method comprises:
  • it is an object of the data processing method to detect the blocking artifacts which originate from a separate coding, via the block-based coding technique of blocks of n ⁇ n pixels, with n 8 in the case of the MPEG standard.
  • the blocking artifact detection method comprises a gradient filter step (GF) of the values (Y) of the pixels contained in the digital input image so as to produce filtered values (G).
  • said gradient filter step utilizes Sobel filters and the values (Y) of the pixels in question are the luminance values.
  • Such filters have been chosen as they ensure both a robust and an efficient contour detection.
  • These filtered values (G) are then compared with threshold values (THR). If they are higher than the threshold values, then a natural contour (NC) will be detected.
  • THR H and vertical threshold values THR V are equal to 35 and 50, respectively, for the luminance values varying from 0 to 255.
  • the blocking artifact detection method also includes a step (CT) which permits to test the continuity or discontinuity of a value of a current pixel relative to the pixels which are adjacent thereto.
  • a vertical discontinuity value c v (i,j) or a horizontal discontinuity value c h (i,j) in the horizontal direction are deduced by the equations (1) and (2), respectively.
  • the step (CT) of testing the discontinuity is followed by a step of determining (BAC) an artifact value of the current pixel based on discontinuity values of the current pixel and of neighboring pixels of the current pixel.
  • FIGS. 2 a and 2 b illustrate the step of determining blocking artifacts for different pixel configurations (Y 1 to Y 5 ).
  • the discontinuity values (C) are also represented for the pixels taken into consideration when the artifact (A) is determined, a discontinuity value equal to 0 being represented by a black square and a value equal to I being represented by a gray square.
  • a first type of artifact A v1 is determined on the basis of the equation (5), which corresponds to FIG.
  • Av 1 ( i,j ) ⁇ overscore ( c h ( i,j ⁇ 1)) ⁇ overscore ( c h ( i,j )) ⁇ ( c h ( i,j+ 1)+ c h ( i,j+ 3)+ c h ( i,j ⁇ 4)+ c h ( i,j ⁇ 2)) (5)
  • ⁇ overscore (c) ⁇ represents the complementary value of c.
  • a second type of artifact A v2 is determined based on the equation (6), which corresponds to FIG. 2 b:
  • Av 2 ( i,j ) c h ( i,j ⁇ 3) ⁇ overscore ( c h ( i,j ⁇ 2)) ⁇ overscore ( c h ( i,j )) ⁇ c h ( i,j+ 2) (6)
  • the result of this determining step is a horizontal or vertical artifact value which is equal to 1 for a pixel having position (i,j) when one or various of the equations (1) to (3) or similar equations leads to determining an artifact (A).
  • the method of detecting blocking artifacts then comprises a step of identifying (ID) blocking artifacts based on previously determined artifact values.
  • ID identifying
  • FIGS. 3 and 4 are diagrams which represent this identification step in a horizontal and vertical direction, respectively.
  • the general idea of this identification step is that a horizontal or vertical block artifact is identified if W consecutive artifacts in a horizontal direction or H consecutive artifacts in a vertical direction have been determined, where H is the height of a block and W its width.
  • a horizontal counter HCV
  • HAC horizontal artifact
  • HCD horizontal counter
  • a horizontal blocking artifact is identified and a table hTab permits to store during a storing step (SCO) a first blocking artifact counter at the position i % H, where i is the line of the image where the horizontal blocking artifact is, % is the operator of which the result is the remainder of the ratio of i to H.
  • SC horizontal blocking artifact at all is identified for the current pixel (n)
  • SC horizontal counter is reset to 0 (HCR) after which the following pixel is tested (SC).
  • a vertical artifact VAC
  • VCI(j) vertical counter
  • the value of this vertical counter is compared with the height H of a block (VAD). If the value of this vertical counter becomes higher than H (y), then a vertical blocking artifact is identified (y) and the table vTab permits to store during a storing step (STO) a second blocking artifact counter at position j % W. If no blocking artifact is identified for the current pixel (n), then the next pixel is tested (SC).
  • the counter is reset to 0 for the column involved (VCR(j)), after which the next pixel is tested (SC). Moreover, the vertical counter is preferably reset to 0 for the column j (VCR(j)) after the storing step, this in order to reduce the storage costs.
  • the tables hTab and vTab permit to deduce the distribution probability of a grid corresponding to the size of the coding blocks.
  • an image belonging to a digital video signal coded according to the MPEG standard comprises blocks of 8 lines and 8 pixels, the first image block starting at position (0,0).
  • an original image belonging to said signal may be shifted by several pixels.
  • the original image may be coded according to various horizontal coding formats so as to keep a good visual quality for low transmission rates. In that case the original image is downsampled horizontally before being coded, and is then upsampled horizontally during the decoding so as to find its initial format back.
  • the result is a modification of the size of the grid due to the upsampling, the coding always being effected on blocks of 8 lines and 8 pixels. If the position and size values of the grid are known during the decoding in a decoder or a set top box, this does not hold for a television receiver that receives an analog signal that does not contain such information.
  • the data processing method comprises a step (GRID) of calculating a position of a grid corresponding to the blocks of the block-based coding technique from a majority position of the block artifacts in the table, and on a grid size from a larger value of the counter values which represent a number of times a distance occurs between a current vertical blocking artifact and an immediately preceding vertical blocking artifact.
  • This calculation step (GRID) of the current size or of the current position of a grid for a current image is carried out as a function of the sizes or preceding positions of the grids determined for the preceding images and as a function of a confidence parameter which is representative of the evolution of the values of said sizes or preceding positions according to the diagrams of FIG.
  • FIG. 4 introduces additional steps represented in dotted lines.
  • the principle of these steps is to determine whether a current vertical blocking artifact is distant from the last vertical blocking artifact stored of 8, 10 or 12 pixels.
  • a general counter is created and then incremented (INC) after the next pixel has been read (SC).
  • STO the value of this general counter is compared with the values 8, 10 and 12 (VAL). If the value of the general counter is equal to 8, 10 or 12, then one of the counters grid 8 , grid 10 and grid 12 which relate to a width of the grid of 8, 10 and 12 pixels, respectively, is incremented (GCI) after which the next pixel is read (SC).
  • the position of the grid (GP) is determined by searching for the most probable values (imax,jmax) in the tables hTab[i] and vTab[j] after a frame or an image has been processed depending on whether the image is interleaved or not.
  • a first test is carried out to know whether the value of a position counter after decrementation is lower than a first predetermined threshold, for example 0. If this is the case (y), then the position counter is set to the value of the first threshold (C 0 ) and the last value of the grid position is used (LAP), which value is initially (0,0), for example; then the frame or the next image is investigated (SCT).
  • a second test is carried out to know whether the value of the position counter after incrementation is higher than a second predetermined threshold, for example 15. If this is the case (y) then the counter is set to the value of the second threshold (C 15 ) and the current value of the grid position is used (CUP); then the next frame or image is investigated (SCT). If the first or second test (T 1 or T 2 ) is not satisfactory (n), then a third test (T 3 ) is carried out to know whether the value of the position counter is higher than a third predetermined threshold, for example 5.
  • the current value of the grid position is used (CUP); if not (n) the last value of the grid position is used (LAP).
  • CUP current value of the grid position
  • LAP last value of the grid position
  • the size of the grid (GP) is determined by searching for the largest value among the values of the counters grid 8 , grid 10 and grid 12 after processing of a frame or of an image according to whether the image is interleaved or not.
  • a size counter is set to 0 (C 0 ) and the last value of the size of a grid is used (LAS), this value being initially 8 ⁇ 8 pixels, for example; then the next frame or image is investigated (SCT).
  • SCT next frame or image is investigated
  • a fourth test (T 4 ) is carried out to know whether the value of the size counter after incrementation is higher than a threshold value fixed at 5 in our example.
  • the size counter forms a confidence parameter which permits to obtain a coherence of the size of the grid during a period of time, the size of the grid can only be modified if 5 consecutive frames give the same grid size.
  • the detection method permits to detect whether a video signal received by a set top box or a simple television receiver has been coded according to a block-based coding technique. Depending on the result of the detection method, corrective actions are then decided on. These actions are, for example, not to apply a particular image processing method, or, in contrast, to apply a post-processing method as a function of the data produced by the detection method.
  • a post-processing method comprises a low-pass filter step (LPF) of the values (y) of the pixels coming from the block artifact detection step (BAD) with the exception of the pixels contained in the natural contour areas (NC) determined by the gradient filter step (GF, THR).
  • LPF low-pass filter step
  • the result of this filter step is an image which contains filtered pixel values (Yf) and whose visual quality is improved relative to the image before processing.
  • FIGS. 7 a and 7 b define the natural contour areas in a horizontal and a vertical direction, respectively. These areas comprise a vertical natural contour (VNC) and several pixels on both sides of this contour (EA) in a horizontal direction; a horizontal natural contour (HNC) and several pixels on both sides of this contour (EA) in a vertical direction.
  • VNC vertical natural contour
  • HNC horizontal natural contour
  • the filters used during this filter step are preferably applied to detected artifacts.
  • the efficiency of the filtering depends in essence on the efficiency of the detection. For this reason, the cost of implementation of the filter step is also reduced.
  • the filter step is completely independent of the block artifact detection step and may consequently be adapted according to the user's wish and the parameters coming from the detection step. Two filter options are proposed here by way of example.
  • filters LP 5 having 5 coefficients and LP9 having 9 coefficients are used for a vertical and a horizontal filtering, respectively.
  • These filters are, for example: LP5 ⁇ ⁇ the ⁇ ⁇ first ⁇ ⁇ and ⁇ ⁇ last ⁇ ⁇ pixel ⁇ ⁇ are ⁇ ⁇ not ⁇ ⁇ filtered the ⁇ ⁇ second ⁇ ⁇ and ⁇ ⁇ last ⁇ - ⁇ but ⁇ - ⁇ one ⁇ ⁇ pixel ⁇ ⁇ are ⁇ filtered ⁇ by ⁇ ⁇ the ⁇ ⁇ filter ⁇ ⁇ 1 4 [ ⁇ 1 2 1 ⁇ ] the ⁇ ⁇ other ⁇ ⁇ pixels ⁇ ⁇ are ⁇ ⁇ filtered ⁇ ⁇ by ⁇ ⁇ the ⁇ filter ⁇ ⁇ 1 16 [ ⁇ 1 4 6 4 1 ⁇ ] ( 9 ) LP9 ⁇ ⁇ the ⁇ ⁇ first ⁇ ⁇ and ⁇ ⁇ last ⁇ pixel ⁇ ⁇ are
  • the filters are variable-length filters defined by the diagrams of FIGS. 8 a and 8 b for a horizontal and a vertical filtering, respectively.
  • the horizontal filter corresponds to the following equations: for ⁇ ⁇ the ⁇ ⁇ pixel ⁇ ⁇ ( i - 4 ) ⁇ : ⁇ ⁇ 1 2 [ ⁇ 1 1 ⁇ ] ⁇ ⁇ for ⁇ ⁇ the ⁇ ⁇ pixel ⁇ ⁇ ( i - 3 ) ⁇ : ⁇ ⁇ 1 4 [ ⁇ 1 1 1 1 ⁇ ] ⁇ ⁇ for ⁇ ⁇ the ⁇ pixel ⁇ ⁇ ( i - 2 ) ⁇ : ⁇ ⁇ 1 8 [ ⁇ 1 1 2 2 1 1 ⁇ ] ⁇ ⁇ ... ⁇ ⁇ for ⁇ ⁇ the ⁇ ⁇ pixel ⁇ i ⁇ : ⁇ ⁇ 1 8 [ ⁇ 1 1 1 1 1 1 1 ⁇ ] ⁇
  • FIGS. 2 to 8 are highly diagrammatic, each Figure representing a single embodiment.
  • a Figure shows various functions in the form of separate blocks, this does not exclude that a single software item caries out various functions. This does not exclude either that one function can be carried out by a software unit.
  • a set of instructions contained in a program memory may cause the circuit to carry out various operations described earlier with reference to FIGS. 2 to 8 .
  • the set of instructions may also be loaded in the program memory by reading a data carrier such as, for example, a disc that contains the set of instructions. The reading may also be carried out via a communication network such as, for example, the Internet. In that case a service provider will make the set of instructions available to interested parties.

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  • Engineering & Computer Science (AREA)
  • Multimedia (AREA)
  • Signal Processing (AREA)
  • Compression Or Coding Systems Of Tv Signals (AREA)
  • Image Processing (AREA)
  • Facsimile Image Signal Circuits (AREA)
  • Compression Of Band Width Or Redundancy In Fax (AREA)
  • Compression, Expansion, Code Conversion, And Decoders (AREA)
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KR20020053014A (ko) 2002-07-04
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